Studies of the ultrastructure and permeability of the blood-brain barrier in the developing corpus callosum in postnatal rat brain using electron dense tracers.

The ultrastructure of the capillaries and their permeability to lanthanum ions and ferritin in the corpus callosum was examined in postnatal rat brain. In 1 and 7-d-old rats, numerous pinocytotic vesicles were observed in the endothelial cytoplasm of the callosal capillaries. Tight junctions were present between adjacent endothelial cells which were surrounded by an ill defined layer of basal lamina. The latter was almost devoid of astrocytic association in 1 d rats and partially covered by astrocytic end-feet in 7 d rats. Pericytes were a common feature. Amoeboid microglial cells were in direct contact with some parts of the vascular wall. Large extracellular spaces were present around the capillaries. In 14 d rats, the walls of the callosal capillaries became more well developed and were surrounded by a continuous sheath of astrocytic end-feet. The basal lamina became denser and well defined. The pericapillary spaces had diminished. Immunostaining for GFAP confirmed that, with age, the walls of the callosal capillaries became increasingly covered by the astrocytic end-feet. After perfusion with lanthanum, the tracer was deposited on the luminal surface but not in the abluminal side of the endothelial cells; the passage of the tracer was apparently obstructed in the intercellular space by the tight junctions in both the 1 and 14 d rats. When injected intravenously in 1-7 d rats, ferritin was transported across the endothelial cells by transcytosis and consequently taken up by the pericytes and amoeboid microglial cells contacting the basal lamina. In 14 d rats, the injected ferritin was only found in the endothelial cytoplasm. It was concluded that the difference in the capillary permeability to exogenous material in 1-7 and 14 d rats is due to the difference in the activities of the transendothelial transport. The pericytes and amoeboid microglial cells associated with the capillary probably play a role as phagocytes in maintaining the function of the blood-brain barrier by trapping any serum-derived foreign substances with astrocytes having a regulatory role in the formation of the barrier.